Conventionally, an electronic blood pressure measurement device employing the oscillometric method is provided. At this electronic blood pressure measurement device, the internal pressure in a bladder located in a cuff (hereinafter, referred to as cuff pressure) wrapped around a measurement site identified as a portion of a living body is adjusted for calculation of a blood pressure value according to change in internal pressure in the bladder that occurs based on volumetric change of the blood vessel that is pressed at the measurement site (hereinafter, referred to as pressure pulse wave). In such an electronic blood pressure measurement device, it is important that the volumetric change of the blood vessel is accurately reflected as the cuff pressure changes. The bladder has a predetermined maximum volume, and is formed of a stretchable resin material that allows variable volume in a range that does not exceed the maximum volume as air is introduced/discharged.
When the cuff pressure (mmHg) changes during blood pressure measurement, the volumetric change of the blood vessel with respect to the pulsatory motion also changes. In an electronic blood pressure measurement device employing the oscillometric method, the volumetric change of the blood vessel is detected as a pressure pulse wave overlapping with the cuff pressure. The systolic and diastolic blood pressure values are calculated based on the pulse wave envelope formed by the detected pressure pulse waves (a curve formed of a collection of pressure pulse waves). The calculating procedure of the blood pressure and pulse count based on the pulse wave envelope is well known, and details thereof will not be presented here.
During blood pressure measurement, it is desirable that the change in the cuff pressure properly reflects the volumetric change of the artery. Variation in the conveyance sensitivity of the volumetric change of the artery corresponding to the cuff pressure will cause reduction in the accuracy of the blood pressure measurement. In other words, variation in the state of the cuff (how tight the cuff is wrapped around the measurement site (namely, the volume of the bladder), or the circumferential length of the arm at the measurement site where the cuff is wrapped, the softness of the measurement site, and the like) will lead to variation in the level of the pressure change obtained corresponding to the blood pressure volumetric change with respect to the same level.
Cuff compliance (ml/mmHg) is known as one index that can express this conveyance sensitivity. The cuff compliance (Cp=dV/dP) is an index representing the cuff volumetric change (dV) to the cuff pressure change (dP). The conveyance sensitivity becomes lower as cuff compliance Cp becomes higher. In other words, the level of pressure change to volumetric change of the same level becomes smaller as the cuff compliance becomes larger.
FIG. 14 schematically shows the relationship of cuff compliance Cp and the amplitude (mmHg) of the pulse wave signal with respect to the change in cuff pressure. Relationship (A) of FIG. 14 represents the change in cuff compliance Cp to the change in cuff pressure. A straight line segment A corresponds to the case where the rate of cuff volumetric change according to change in the cuff pressure is constant, i.e. the value of cuff compliance Cp is constant (parallel) to the change in cuff pressure. When air is input to or output from the bladder of the cuff, the cuff compliance will change and not become constant to the cuff pressure, as indicated by curve B, different from line segment A, even if the same cuff pressure change is presented.
In relationship (B) of FIG. 14, the change in the amplitude of the pulse wave signal detected concurrently is designated as A1 and B1, when a cuff having a bladder corresponding to the detection of cuff compliance Cp of line segment A and curve B in relationship (A) is wrapped around a measurement site (upper arm). The pulse wave amplitude detected when cuff compliance Cp is constant with respect to the cuff pressure (line segment A) is designated A1. The pulse wave detected when cuff compliance Cp varies with respect to the cuff pressure (curve B), i.e. when the volumetric change rate of the bladder of the cuff is not constant, is designated B1.
The pulse wave amplitude represents the volumetric change of the blood vessel pressed by the cuff. In the case where the blood vessel volumetric change is conveyed without lost via the cuff and detected by a pressure sensor or the like, the blood pressure can be measured accurately. However, when cuff compliance Cp varies with respect to the cuff pressure as in curve B, the detected pulse wave amplitude indicating the pulse wave component will be distorted due to the variation. Therefore, distortion occurs in the pulse wave envelope corresponding to a series of such pulse wave signals.
Distortion of the pulse wave amplitude is exhibited such that the amplitude is increased at the high level side of the cuff pressure and reduced at the low level side of the cuff pressure, respectively. A high cuff pressure means that the bladder is sufficiently inflated by the great amount of air introduced therein. Therefore, the amplitude of the pressure pulse wave indicating the volumetric change of the blood vessel pressed by the cuff is distorted to become larger than the pressure pulse wave amplitude indicating the actual volumetric change value of the blood vessel. In contrast, when the cuff pressure is low, the amount of air in the bladder is low. Therefore, the pressure pulse wave amplitude indicating the volumetric change of the blood vessel pressed by such a cuff is distorted to become smaller. Thus, the blood pressure measurement accuracy is degraded by the distortion component set forth above in the event of air being input to and output from the bladder, as in curve B.
FIG. 15 represents the relationship of cuff compliance Cp and the pulse wave amplitude according to cuff pressure change when the cuff is wrapped around a measurement site (upper arm), corresponding to the arm size at the measurement site (length around the arm). In relationship (A) of FIG. 15, the relationship between cuff compliance Cp and the pulse wave amplitude is represented by curves A and B corresponding to the case where the circumferential length of the arm is long and short, respectively.
In relationship (B) of FIG. 15, the pulse wave amplitude change detected when the cuff pressure change is as in relationship (A) is indicated by a pulse wave signal designated A1 and a pulse wave signal designated B1. The pulse wave signal of A1 corresponds to the case where cuff compliance Cp changes as in curve A. The pulse wave signal of B1 corresponds to the case where cuff compliance Cp changes as in curve B. As indicated in the drawings, since the volume of the bladder in the wrapped cuff is larger for the longer circumferential length than for the shorter circumferential length, the volumetric change (volumetric change rate) in the bladder required to achieve a predetermined cuff pressure will be larger for the longer circumferential length than for the shorter circumferential length, resulting in a detected pulse wave amplitude smaller for the longer circumferential length than for the shorter circumferential length.
In addition, the ratio of cuff compliance Cp differs between the high side and low side of the cuff pressure, depending upon the arm thickness. In other words, cuff compliance Cp ratio b2/b1 of the high pressure side to the low pressure side at a small arm size of curve B differs from cuff compliance Cp ratio a2/a1 for a large arm size of curve A. Therefore, the detected pulse wave amplitude is greatly distorted depending upon the arm thickness.
FIG. 16 represents the relationship of cuff compliance Cp and the pulse wave amplitude according to cuff pressure change when the cuff is wrapped around a measurement site (upper arm), corresponding to the softness of the arm (soft/firm) at the measurement site. In relationship (A) of FIG. 16, the relationship between cuff compliance Cp and the pulse wave amplitude is represented by curve C and curve D corresponding to a soft arm and a firm arm, respectively.
In relationship (B) of FIG. 16, the pulse wave amplitude change detected when the cuff pressure change is as in relationship (A) is indicated by a pulse wave signal designated C1 and a pulse wave signal designated D1. The pulse wave signal of C1 corresponds to the case where cuff compliance Cp changes as in curve C. The pulse wave signal of D1 corresponds to the case where cuff compliance Cp changes as in curve D. As indicated in the drawings, the required air volume of the cuff to achieve the same cuff pressure will be larger if the measurement site (arm) is soft than if the arm is firm, resulting in a detected pulse wave amplitude smaller for the soft arm than for the firm arm. In addition, the ratio of cuff compliance Cp differs between the high side and low side of the cuff pressure, depending upon the softness of the arm. In other words, cuff compliance Cp ratio d2/d1 of the high pressure side to the low pressure side at a firm arm of curve D differs from cuff compliance Cp ratio c2/c1 for a soft arm of curve C. Therefore, the detected pulse wave amplitude is greatly distorted depending upon the arm softness.
Thus, the different cuff compliance ratio between a soft arm and a firm arm according to the cuff pressure will cause the pressure pulse wave amplitude to be distorted. Therefore, the accuracy of the blood pressure measurement will vary depending upon the soft/firm arm.
FIG. 17 represents the relationship of cuff compliance Cp and the pulse wave amplitude according to cuff pressure change when the cuff is wrapped around a measurement site (upper arm), corresponding to the wrapping tightness of the cuff at the measurement site. In relationship (A) of FIG. 17, the relationship between cuff compliance Cp and the pulse wave amplitude is represented by curve E and curve F corresponding to a tightly wrapped case and loosely wrapped case, respectively.
In relationship (B) of FIG. 17, the pulse wave amplitude change detected when the cuff pressure change is as in relationship (A) of FIG. 17 is indicated by a pulse wave signal designated E1 and a pulse wave signal designated F1. The pulse wave signal of E1 corresponds to the case where cuff compliance Cp changes as in curve E. The pulse wave signal of F1 corresponds to the case where cuff compliance Cp changes as in curve F.
As appreciated from the drawing, in the case where the cuff is wrapped loosely around the measurement site, an amount of air that allows blood pressure measurement, even if introduced sufficiently into the bladder of the cuff, will need to be further increased in the bladder to actually press the cuff against the measurement site. This means that the amount of air volume to be introduced into the cuff bladder is increased as compared to the case where the cuff is wrapped tightly in order to raise the cuff pressure to the same level. Thus, the amount of air to be introduced in the cuff bladder in order to raise the cuff pressure to the same level is increased in the state where the cuff is wrapped loosely as compared to the state where the cuff is wrapped tightly or appropriately. Thus, the detected pressure pulse wave amplitude becomes smaller in a loosely wrapped state as compared to the tightly or appropriately wrapped state even if the cuff pressure is the same.
In contrast, when the cuff is wrapped tightly, the required amount of air to be introduced into the bladder in order to raise the cuff pressure to the same level is smaller as compared to a loosely wrapped state. Therefore, the detected pressure pulse wave is larger than in a loosely wrapped state. Thus, the level of the pulse wave amplitude differs as indicated by E1 and F1 in relationship (B) of FIG. 17 even if the cuff pressure is the same, depending upon the state of wrapping tightness (wrapped tightly or loosely) around the measurement site. Similarly as described above, cuff compliance Cp ratio differs between the high side and low side of the cuff pressure, depending upon the wrapping state. Cuff compliance Cp ratio e2/e1 of the high pressure side to the low pressure side differs from cuff compliance Cp ratio f2/f1 in a tightly wrapped state of curve F, so that the pulse wave is distorted due to the cuff volumetric change ratio not being constant (refer to relationship (A) of FIG. 17). Thus, the accuracy of blood pressure measurement will be degraded due to the wrapping state.
As shown in FIGS. 14-17, change occurs in the pulse wave amplitude corresponding to the volumetric change of the blood pressure when the cuff state (softness of arm, circumferential length of arm, cuff wrapping tightness) changes. Moreover, the pulse wave amplitude will change if cuff compliance Cp differs. Thus, even if the artery is squeezed with the same cuff pressure, the detected pulse wave amplitude will vary, i.e. be distorted, depending upon the cuff state and difference in cuff compliance Cp.
Conventional approaches of blood pressure measurement taking into account the cuff state and compliance are disclosed in patent documents.
Japanese Patent Laying-Open No. 5-329113 discloses the method of measurement including the steps of identifying in advance the cuff volumetric change property with respect to the cuff pressure, converting a signal of the cuff pressure change to volumetric change, and correcting the blood pressure value using the same for measurement. In accordance with this method, the cuff pressure and volumetric change property must be prepared in advance.
Japanese Patent Laying-Open Nos. 11-309119 and 11-318835 disclose a sphygmomanometer cuff including pressing means for supplying a predetermined amount of fluid to a fluid bladder for squeezing the human body to press the fluid bladder against the living body.
Japanese Patent Laying-Open No. 5-269089 discloses a sphygmomanometer cuff including a small inner cuff into which a conductive liquid of low viscosity is introduced for pressing against an artery, configured to press the inner cuff against a human body using an outer cuff located at the outer side of the inner cuff.    Patent Document 1: Japanese Patent Laying-Open No. 5-329113    Patent Document 2: Japanese Patent Laying-Open No. 11-309119    Patent Document 3: Japanese Patent Laying-Open No. 11-318835    Patent Document 4: Japanese Patent Laying-Open No. 5-269089